Stepping inductor for fast transient response of switching converter

ABSTRACT

A fast transient response converter is disclosed which makes use of stepping inductor in a switching converter to speed up output voltage response under fast transient condition. The inductive element in a switching converter is replaced by two series or parallel inductive elements, one of which has a smaller value of inductance than the other. During the fast transient period, the inductor with larger inductance value will be shorted to a voltage source. The total inductance will be greatly reduced and thus allows rapid current change during the transient change.

1. FIELD OF THE INVENTION

[0001] This invention relates to the field of switch mode powerconverters, and in particular fast dynamic response of power converters.

2. BACKGROUND OF THE INVENTION

[0002] Switching converters are widely used to provide high efficiencyand light weight power supply, however the transient output response toa fast load change is inherently limited by the output inductor that ispresent in most switching power converters. A typical prior art buckconverter comprises a power stage with a plurality of switches, aninductor-capacitor filter and a feedback circuit. This feedback circuitmonitors the converter output voltage and controls Pulse WidthModulation (PWM) of the switches. When there is a fast dynamic loadchange the response of the converter is limited by two parts of theconverter, namely the feedback circuit and the power stage. The feedbackcircuit can be designed to be very fast by traditional linear approachor non-linear approach. However, the inherent response of the converteris limited by the output filter which is largely determined by the valueof the output inductor.

[0003] Many researchers in the field have attempted to enable fastresponse through several different methods but all of the proposedsolutions have limitations. Some researchers have attempted to speed uppower converter dynamic response by using an inductor with smallinductance value. This approach seemed to solve the problem becausecurrent delivery can rise much more rapidly through a small inductor.However, this approach brought on the problem of excessively highcurrent ripple during normal operation, which introduces high root meansquare current in the converter switches and passive components and thisincreases power loss. Other researchers have attempted to reduce lossesby using parallel multiphase converters to share the current but thisresults in increased cost and complexity. Yet other researchers haveattempted to address the problem by increasing the switching frequency.However, this introduces the problem of excessive switching losses inswitches and magnetic losses in the inductor core. Additionally, highfrequency operation requires high performance drive circuit whichfurther escalates cost.

[0004] There is therefore an acute need for a method to provide fastresponse while maintaining a low loss level of the converter and atreduced cost for computer applications.

[0005] An invention U.S. Pat. No. 6,188,209 provides the basis for thepresent invention. The present invention further reduces complexity andprovides alternatives to achieve fast transient response.

3. SUMMARY OF THE INVENTION

[0006] The present invention discloses different embodiments of anapparatus and a method with many salient features that provides fasttransient response of switching power converters. The present inventiondramatically increases the rate of change of current through an outputinductor in a converter during transient while maintaining low currentripple at normal load. As it is not necessary to practice the presentinvention at high frequency, converter loss is kept to a minimum.However, applicant does not exclude the possibility of operating at highfrequencies. The apparatus disclosed herein is operational with mostpower converter with an output inductor.

[0007] The basic approach of the disclosed method is to replace theinductor of a switching converter with one or more inductors with ahigher inductance that operates during steady loading condition, and thecapabilities to switch to one or more inductors with a lower inductanceduring a fast transient loading condition. This is accomplished byreplacing the output inductor of a conventional buck switching converterwith at least two series inductors, one of which has a small inductance,while the other has much higher inductance. The two terminals of theinductor with the higher inductance are programmed to be connected to avoltage source during transient condition. The voltage source canprovide a rapid change of current in the output inductor with higherinductance while the inductor has been shorted to the voltage source.The connection to the voltage source reduces total equivalent seriesinductance of the two series inductor to the inductance of the inductorwith small inductance, and enables high rate of change of current to theoutput load.

[0008] The voltage source used to short out the inductor can be anyvoltage in the converter, e.g. input voltage, output voltage or voltagedrop of a switch or diode.

[0009] This invention produces low inductor ripple current in the steadystate. In a specific embodinemt, during steady state operation, theequivalent series inductance of the series inductors is the summation ofthe two inductors. The inductor with high inductance is designed to belarge enough to maintain very small ripple current to minimize the rootmean square (RMS) current flowing through the switching elements andother components. The inductor with small inductance is designed to besmall enough to provide necessary rate of change of current when theinductor with higher inductance is shorted out by a voltage sourceduring transient condition. The transient conditions only exist for ashort period of time and a converter spends most of its operating timein the steady state. Hence the converter will carry high ripple currentonly for a short duration of time and efficiency will not be seriouslyimpaired. This invention is versatile and can be applied to mostswitching converters with output inductor.

[0010] In an alternative embodiment, two parallel inductors, one ofwhich has a high inductance and the other having a much lowerinductance, can also be employed similar to the description above, inproviding large inductance during steady loading condition and switch tolower inductance during fast transient load change.

[0011] Accordingly, it is an object of the present invention to providefast dynamic response to switching power converters.

[0012] It is another object of the present invention to maintain lowoutput inductor ripple current.

[0013] It is another object of the present invention to improveconverter dynamic response without operating at very high frequency.

[0014] It is another object of the present invention to maintain highconverter efficiency.

[0015] It is another object of the present invention to use a simplecontrol method.

[0016] These and other objects of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of the invention and from the accompanying drawings.

4. BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 (Prior Art) illustrates a simplified equivalent circuit ofa prior art Buck converter.

[0018] FIGS. 2A-2D illustrate a waveform diagram during a load transientfor the prior art Buck converter shown in FIG. 1.

[0019]FIG. 3 illustrates the basic operation of a first embodiment ofthe present invention.

[0020]FIG. 4 illustrate waveforms for the first embodiment of thepresent invention during transient load current increase.

[0021]FIG. 5 illustrate waveforms for the first embodiment of thepresent invention during transient load current decrease.

[0022]FIG. 6 illustrates a second embodiment of the present invention.

[0023]FIG. 7 illustrates a third embodiment of the present invention.

[0024]FIG. 8 illustrates an isolated converter of the fourth embodimentof the present invention.

[0025]FIG. 9 illustrates a fifth embodiment of the present invention.

[0026]FIG. 10 illustrate waveforms for the fifth embodiment duringtransient load current increase.

[0027]FIG. 11 illustrates waveforms for the fifth embodiment duringtransient load current decrease.

5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The detail descriptions of the present invention have been givenin relation to the prior art buck converter. However, it is obvious toone of skill in the art that the present invention is not limited tobuck converters but can also be applied to other converters. FIG. 1illustrates a prior art buck converter with two switches and an outputinductor capacitor filter. FIG. 2 illustrates that when there is astep-like rise in load current, as illustrated in FIG. 2C, assuming thefeedback circuit 4 and the Pulse Width Modulation controller 6illustrated in FIG. 1, are fast enough to change the duty cycles of theconverter switches to enable the inductor current to rise to a newaverage (FIG. 2D). The rate of increase of the current is still limitedby the inductance of the output inductor. A small inductance would allowa fast rate of change of current but the converter still suffers from ahigh ripple inductor current. A large inductance would reduce ripplecurrent but the inductor current will change slowly. The presentinvention provides several novel embodiments that tackle this problem ofallowing fast inductor current rate changes while at the same timeproviding a way of reducing the ripple current.

[0029] First Embodiment

[0030]FIG. 3 illustrates basic the circuit of the first embodiment ofthe invention, which comprises a power circuit and a control circuit.The power circuit comprises a pair of input terminals 101 and 102 forconnection to a DC voltage source. It further comprises a pair ofswitches, represented by MOSFETs M101 and M102, which produce a seriesof alternating voltage pulses at node 130. This pair of switches iscoupled to a transformer T101, which comprising windings W101, W102 andW103, and an output capacitor C101, as indicated in FIG. 3. A load isconnected to the output capacitor C101. Winding W101 is coupled directlyto capacitor C101. Winding W102 is coupled to the input voltage sourceand the voltage produced by the voltage drop of the MOSFET S104 anddiode D101. Winding W103 is coupled to an input voltage source and thevoltage source produced by the voltage drop of the MOSFET S103 and diodeD102. Two MOSFETs S103 and S104 control the connection of voltage sourceacross windings W103 and W102 respectively. Two diodes D101 and D102block the reverse voltage across the MOSFETs S103 and S104 and thediodes also provide additional voltage.

[0031] Most power devices attached to the converter are low voltagedevices. MOSFET S104 will sustain a lower voltage at turn off, which isequal to the difference between the input voltage and the voltagegenerated by winding W102, when MOSFET M101 is turned on. MOSFET S103also sustain a lower voltage at turn off, as the voltage across windingW103 is determined by the winding ratio and output voltage, and theoutput voltage is always lower than the input voltage when MOSFET M102is turned on. Hence a low voltage MOSFET can be used. Similarly diodeD101 sustain a lower reverse voltage, as the voltage across winding W102is determined by the winding ratio and output voltage and the outputvoltage is always lower than input voltage, when MOSFET M102 is turn on.Diode D102 sustain a lower reverse voltage, as no extra voltage sourceis added to increase the reverse voltage, when MOSFET M101 is turned on.All these additional low voltage devices will therefore not increaseproduct cost significantly.

[0032] The control circuit of the first embodiment comprises a feedbackblock 104, which is then coupled to a Pulse Width Modulation (PWM) block106, and which provides driving pulses to the switches in the powercircuit. A plurality of pulses are then produced between node 130 andnode 102. The plurality of pulses will then be filtered by inductanceacross winding W101 and output capacitor C101 to produce a regulated DCoutput.

[0033] The feedback block monitors converter load voltage and the PWMblock drives the power switches M101 and M102 in a closed loop. Thereare a plurality of apparatuses which form a second loop which monitorsconverter load voltage and produce driving signals for switches S103 andS104 in the power circuit. These apparatus comprise two hysteresiscomparators B102 and B103 which form a sensing circuit to monitorconverter load voltage, and are coupled to switches S104 and S103respectively.

[0034] During operation, feedback block 104 monitors the converteroutput voltage and generates signal to control the PWM controller 106,which in turn generates gate pulses to drive MOSFETs M101 and M102 andmaintains a steady output voltage across the load 107. During steadystate operation, the two switches S103 and S104 are opened, hence theinductance of W101 provides high inductance to keep the ripple currentlow.

[0035] The waveforms of FIG. 4 illustrate the operation of the presentinvention when there is a fast transient increase in load current. Inthe period between t10 and t11 the converter operates in steady state.At time t11 there is a step increase in load current as illustrated inFIG. 4C. This leads to output voltage drop as illustrated in FIG. 4E.When the voltage drops below a threshold level V1 switch S104 isswitched on by the apparatus B102. Inductance of winding W101 diminishesand the equivalent inductance drops to the level of leakage inductance.This enables current to rises rapidly as illustrated in FIG. 4D. In thetime period between t12 and t13 current flows through winding W102 aswell. This current consists of the reflected current from winding W101and magnetizing current which is energized by the input voltage source.The magnitude of this current is dependent on the ratio of the number ofturns of windings W101 and W102. In this time period the magnetizingcurrent of transformer T101 increases as a result of the imposed inputvoltage across winding W102. The increase in current raises the outputvoltage until it reaches a second voltage level V2 at time t13 andswitch S104 is turned off.

[0036] The input voltage source is not necessarily the only voltage tobe applied across winding W102 or W103 during transient loadingcondition, and any voltage source found in the converter can be used,e.g. the voltage drop across the switches S104 or S103.

[0037] The output voltage recovers and reaches a preset reference V2. Attime t13 switch S104 is turned off and current through winding W102falls to zero during the time period t13 to t14. When switch S104 isturned off the magnetizing current of transformer T101 will be coupledto winding W101. At time t13, the current through the leakage inductorof T101 and the magnetizing current through W101 are not necessarily thesame. The difference in current will charge up the stray capacitance ofswitch S104 and create a voltage spike in the time period t13 to t14 asillustrated in FIG. 4H. An energy absorption circuit, such as a snubbercircuit, can be employed to avoid overvoltage in winding W101. Suchenergy absorption circuit may be coupled to winding W103, W102,transformer T101, switch S103 or switch S104.

[0038] After the fast transient, the converter resumes normal operationquickly. The current through winding W102 drops to zero at time t14.Beyond time t14 the equivalent series output inductance resumes theinductance of W101. The output inductor current now changes at a smallerslope. If the inductor current matches with the required load current,the output voltage will rise until feedback circuit 104 operates toresume normal pulse width modulation. However, there is a chance thatthe inductor current at time t14 falls short of the required loadcurrent and the output voltage falls after switch S104 is turned off att14. In this event, the voltage level may fall back to the level V1 andthe whole process of triggering switch S104 on will repeat. If S104 istriggered on again the inductor current through W101 will rise until itis sufficient to maintain the load current. Hence the output voltagewill eventually rise to a level which resumes normal pulse widthmodulation.

[0039] The waveforms of FIG. 5 illustrate the operation of the presentinvention when there is a fast transient decrease in load current. Inthe time period t20 to t21 the converter operates with steady loadcurrent. At time t21 the load current experiences a step-like decreaseas illustrated in FIG. 5C. This causes an increase in output voltage, asillustrated in FIG. 5E. Assuming that feedback circuit 104 and PWMcontroller 106 are fast enough to turn off MOSFET M101 and turn onMOSFET M102, the current reduction in W101 is still slow because of thehigh inductance of W101. At time t22, the output voltage reaches a levelV3 which triggers switch S103 to turn on through B103. Effectivelywinding W103 is shorted by switch S103 and D102, the inductance of W101disappears and the rapid decrease in current is taken up by the currentin winding W103. A magnetizing current is also established in windingW103, caused by the voltage drop of switch S103 and D102. As a result ofthe reduction in current, the output voltage decreases until it reachesanother voltage level V4 at time t23. This triggers switch S103 and themagnetizing current is transferred to winding W101. This magnetizingcurrent does not necessarily match the current flow in leakage inductor.This will cause a voltage spike across winding W103 in the time periodt23 to t24 as illustrated in FIG. 5H. An energy absorption circuit, suchas a snubber circuit, can be employed to avoid overvoltage in windingW101. Such energy absorption circuit may be coupled to inductor W103,W102, transformer T101, switch S103 or switch S104.

[0040] In the time period after t24, the output voltage graduallyreduces to an appropriate level such that the normal feedback loop andPWM controller resume normal operation. However, there is still apossibility that current through the leakage inductor L101 has notsufficiently decreased to prevent the output voltage from reachingvoltage threshold level V3 after time t24. In this case the outputvoltage will eventually hit voltage triggering level V3 and the processwill repeat until the output voltage reaches a steady state.

[0041] The present invention provides means to keep the output voltageof the converter within certain limits and is able to provide fasttransient response against sudden load current changes.

[0042] Second Embodiment

[0043]FIG. 6 illustrates a second embodiment of the present invention,which synchronizes the switch on and switch off time of switches M201,N202, S203 and S204, in order to achieve faster response. Thisembodiment also comprises a power circuit and a control circuit, similarto the first embodiment.

[0044] The power circuit comprises a pair of input terminals 201 and 202for connection to a DC voltage source, and further comprises a pair ofswitches, represented by MOSFETs M201 and M202, which produces a seriesof alternating voltage pulses at node 230. This pair of switches iscoupled to a transformer T201, which comprises windings W201, W202 andW203, and an output capacitor C201, as indicated in FIG. 6. A load isconnected to the output capacitor C201, and winding W201 is coupleddirectly to capacitor C201. Windings W202 is coupled to a input voltagesource and the voltage source caused by the voltage drop of the MOSFETS204 and diode D201. Windings W203 is coupled to the voltage sourcecaused by the voltage drop of the MOSFET S203 and diode D202. TwoMOSFETs S203 and S204 control the connection of voltage source acrosswindings W203 and W202 respectively. Two diodes D201 and D202 block thereverse voltage across the MOSFETs S203 and S204 and the diodes alsoprovide an alternative voltage source to short the windings.

[0045] Most devices attached to the power converter are low voltagedevices. MOSFET S204 will sustain a lower voltage than the input at turnoff, which is equal to the difference between the input voltage and thevoltage generated by winding W202 when MOSFET M201 is turn on. MOSFETS203 also sustains a lower voltage at turn off, as the voltage acrosswinding W203 is determined by the ratio of the winding and the outputvoltage is always lower than input voltage, when MOSFET M202 is turn on.Hence a low voltage MOSFET can be used. Similarly diode D201 sustains alower reverse voltage, as the voltage across winding W202 is determinedby the winding ratio and the output voltage, and the output voltage isalways lower than input voltage when MOSFET M202 is turn on. Diode D202sustain a lower reverse voltage, as no extra voltage source is added toincrease the reverse voltage, when MOSFET M201 is turn on. As these areall low voltage devices, they will not significantly increase productcost.

[0046] The control circuit of the second embodiment comprises a feedbackblock 204, which is then coupled to a Pulse Width Modulation (PWM) block206, which provides driving pulses to the switches in the power circuit.A plurality of pulses is then produced between node 230 and node 202.The plurality of pulses will then be filtered by inductance acrosswinding W201 and output capacitor C201 to produce a regulated DC output.

[0047] The feedback block monitors converter load voltage and the PWMblock drives the power switches M201 and M202 in a closed loop. Thereare a plurality of apparatuses which form a second loop, which monitorsconverter load voltage, and produce driving signals for switches S203and S204 in the power circuit. These apparatuses comprise two hysteresiscomparators B202 and B203 which form a sensing circuit to monitorconverter load voltage. These comparators are coupled switches S204 andS203 respectively.

[0048] In order to give the fastest transient response of the converter,a logic circuit comprising IC201, IC202, IC203, IC204, IC205 and IC206is present to ensure that MOSFET M201 turns on under any condition whenauxiliary switch S204 is triggered by B202 to turn on. This overridesthe slower feedback circuit 204 and PWM controller 206. The logiccircuit ensures MOSFET M202 turns on under any condition when auxiliaryswitch S203 is triggered by B203 to turn on. If both S203 and S204 arenot triggered by B203 and B202, MOSFETs M201 and MOSFET M202 will bedriven by the signal from the PWM controller 206.

[0049] The steady state and transient operation of this embodiment isthe same as that of the first embodiment. Appropriate turns ratios ofwindings in transformer T201 are used.

[0050] Third Embodiment

[0051]FIG. 7 illustrates a third embodiment of the present invention,which eliminates the need for the transformer to carry both the steadystate output current and transient current. The steady state current ishandled by a parallel inductor while the transient current is handled bya separate transformer. This increases the flexibility for theconstruction of the inductor and allows better control of parameters.This embodiment also comprises a power circuit and a control circuitsimilar to the first embodiment.

[0052] The power circuit comprises a pair of input terminals 301 and 302for connection to a DC voltage source, and further comprises a pair ofswitches, represented by MOSFETs M301 and M302, which produces a seriesof alternating voltage pulses at node 330. This pair of switches iscoupled to a transformer T301, which comprises windings W301, W302 andW303, and an output capacitor C301, as illustrated in FIG. 7. A load isconnected to the output capacitor C301, and winding W301 and inductorL301 are coupled directly to capacitor C301. Winding W302 is coupled toan input voltage source and the voltage produced by the voltage drop ofthe MOSFET S304 and diode D301. Winding W303 is coupled to the voltagesource produced by the voltage drop of the MOSFET S303 and diode D302.Two MOSFETs S303 and S304 control the connection of voltage sourceacross windings W303 and W302 respectively. Two diodes D301 and D302block the reverse voltage across the MOSFETs S303 and S304 and thediodes also provide an alternative voltage source to short the windings.

[0053] Most power devices attached to the converter are low voltagedevices. MOSFET S304 will sustain a lower voltage at turn off, which isequal to the difference between the input voltage and the voltagegenerated by winding W302, when MOSFET M301 is turned on. MOSFET S303also sustain a lower voltage at turn off, as the voltage across windingW303 is determined by the winding ratio and output voltage, and theoutput voltage is always lower than input voltage when MOSFET M302 isturn on. Hence a low voltage of the MOSFET can be used. Similarly diodeD301 sustain a lower reverse voltage, as the voltage across winding W302is determined by the winding ratio and output voltage and the outputvoltage is always lower than input voltage, when MOSFET M302 is turn on.Diode D302 sustain a lower reverse voltage, as no extra voltage sourceis added to increase the reverse voltage, when MOSFET M301 is turn on.These are all low voltage devices, which will not significantly increaseproduction costs.

[0054] The control circuit comprises a feedback block 304 which is thencoupled to a Pulse Width Modulation (PWM) block 306, which providesdriving pulses to the switches in the power circuit. A plurality ofpulses is then formed between node 330 and node 302. The plurality ofpulse will then be filtered by the inductance across inductor L301 andoutput capacitor C301 to form a regulated DC output.

[0055] The feedback block monitors converter load voltage and the PWMblock 306 drives the power switches M301 and M302 in a closed loopmanner. There are a plurality of apparatuses which form a second loop,which monitors converter load voltage, and produce driving signals forswitches S303 and S304 in the power circuit. These apparatuses comprisetwo hysteresis comparators B302 and B303 which form a sensing circuit tomonitor converter load voltage. These comparators are coupled toswitches S304 and S303 respectively.

[0056] In order to give the fastest transient response of the converter,a logic circuit comprising IC301, IC302, IC303, IC304, IC305 and IC306,ensures MOSFET M301 turns on under any condition when auxiliary switchS304 is triggered by B302 to turn on, or to ensure MOSFET M302 turns onunder any condition when auxiliary switch S303 is triggered by B303 toturn on. If both S303 and S304 are not triggered by B303 and B302,MOSFETs M301 and MOSFET M302 will be driven by the signal from the PWMcontroller 306.

[0057] The operation of this embodiment is the same as that of thesecond embodiment, except that the steady state output current flows inan external inductor L301, rather than in the magnetizing inductance, asin winding W303. Hence the transformer T301 can be made smaller, as itis used in transient loading condition. Appropriate turns ratio ofwindings in transformer T301 are used.

[0058] Inductor L301 can be made of one single winding on a magneticcomponent, or it can be a combination of several individual smallerinductors in parallel or in series for ease of making under high currentapplication.

[0059] Fourth Embodiment

[0060] The present invention can also be applied to isolated powerconverters. A fourth isolated converter embodiment of the presentinvention is illustrated in FIG. 8. A plurality of pulses comes from theoutput winding and its corresponding rectification circuit. Thisembodiment is somewhat similar to the previously presented embodiments,in that it comprises a power circuit and a control circuit. However, itfurther comprises an isolation and rectification circuit.

[0061] The power circuit comprises a pair of input terminals 401 and 402for connection to an isolating DC-AC converter and output rectificationunit. The output of the isolating DC-AC converter and outputrectification unit is connected across node 430 and node 409, andproduces a series of alternating voltage pulses at node 430. Thisplurality of pulses is coupled to a transformer T401, which compriseswindings W401, W402 and W403, and an output capacitor C401, as indicatedin FIG. 8. A load is connected to the output capacitor C401, and windingW401 is coupled directly to capacitor C401. Windings W402 is coupled toa voltage produced by the voltage drop of the MOSFET S404 and diodeD401. Windings W403 is coupled to a input voltage source and the voltagesource caused by the voltage drop of the MOSFET S403 and diode D402. TwoMOSFETs S403 and S404 control the connection of voltage source acrosswindings W403 and W402 respectively. Two diodes D401 and D402 block thereverse voltage across the MOSFETs S403 and S404 and the diodes alsoprovide an alternative voltage source to short the windings.

[0062] The control circuit comprises a feedback block 404 which is thencoupled to a Pulse Width Modulation (PWM) block 406 which providesdriving pulses to the switches in the power circuit. A plurality ofpulses is then formed between node 430 and node 402. The plurality ofpulses will then be filtered by inductance across winding W401 andoutput capacitor C401 to form a regulated DC output.

[0063] The feedback and isolation block 404 monitors converter loadvoltage and the PWM block 406 giving signal to control the duty cyclegenerated at node 430. There are a plurality of apparatuses which form asecond loop, which monitors converter load voltage and produce drivingsignals for switches S403 and S404 in the power circuit. Theseapparatuses comprise two hysteresis comparators B402 and B403 which forma sensing circuit to monitor converter load voltage. These comparatorsare coupled switches S404 and S403 respectively.

[0064] This converter does not have a steady voltage source, thereforeappropriate control has to be applied. In order to provide the fastesttransient response of the converter, a logic circuit comprising IC401,IC402, and IC403, ensures auxiliary switch S404 is triggered by B402 toturn on when the pulse voltage at node 430 is high, or to ensureauxiliary switch S403 is triggered by B403 to turn on when the pulsevoltage at node 430 is low.

[0065] The operation of this embodiment is the same as that of thesecond embodiment except the plurality of pulse at node 430 is notgenerated by the series MOSFETs but from the isolating DC-AC converterand output rectification.

[0066] Fifth Embodiment

[0067] A fifth embodiment of the present invention is illustrated inFIG. 9. In this embodiment the principle of stepping inductance isdifferent from all of the aforementioned embodiments. A small inductorand a large inductor are arranged in parallel and a switch is connectedin series with the small inductor. The switch is normally open toisolate the small inductor. When there is a transient change in loadvoltage the switch is closed to connect the small inductor in parallelwith the large inductor and enables fast current change. FIG. 9illustrates this embodiment, which also comprises a power circuit and acontrol circuit.

[0068] The power circuit comprises a pair of input terminals 501 and 502for connection to a DC voltage source, and further comprises switches,represented by MOSFETs M501 and M502, which produces a series ofalternating voltage pulses. This pair of switches is coupled to aninductor L502 which is further coupled to an output capacitor C501 asindicated in FIG. 9. An inductor L501 with a series switch made up oftwo series unidirectional switches S503 and S504 are coupled in parallelwith inductor L502. Two diodes D503 and D504 are coupled to the nodeconnecting switch S503, S504 and inductor L501 for voltage clampingpurpose and protection of switches S503, S504. A load is connected tothe output terminals attached to output capacitor C501.

[0069] The control circuit comprises a feedback block 504, which is thencoupled to a Pulse Width Modulation (PWM) block 506, which providesdriving pulses to the switches in the power circuit. The feedback blockmonitors converter load voltage and the PWM block 506 drives the powerswitches M501 and M502 in a closed loop manner. There are a plurality ofapparatuses which form a second loop, which monitors converter loadvoltage, and produce driving signals for switches S503, S504 in thepower circuit. These apparatuses comprise a high pass filter B501 whichmonitors converter load voltage and is coupled to two hysteresiscomparators B502 and B503. These comparators are coupled to AND gatesIC504 and IC 503. Driving signals for MOSFETs M501 and M502 are alsoinput signals to these AND gates. The outputs of these AND gates are fedinto an OR gate IC505 which drives switch S503 on and off accordingly.

[0070] The steady state operation is explained. Feedback block 504generates a signal to control the PWM controller 506, which generatesgate pulses to drive MOSFETs M501 and M502 and maintains a steadyvoltage across the load 507. The operation is the same as that of aconventional converter with an output inductor L502 and output capacitorC501. During steady state operation, switches S503, S504 are open sothat inductor L501 is not involved in power conversion. Inductor L502has inductance high enough to suppress excessive ripple current. Thismaintains high efficiency during steady load condition. Inductor L501has inductance considerably smaller than that of inductor L502.

[0071] When there is a fast transient increase in load current, thepresent converter tackles the transient illustrated by waveformsillustrated in FIG. 10. In the period between t30 and t31 the converteroperates in steady state. At time t31 there is a step increase in loadcurrent as illustrated in FIG. 10C. This leads to an output voltagedrop, as illustrated in FIG. 10E. Even under the assumption thatfeedback circuit 504 and PWM controller 506 is fast enough to turn onMOSFET M501 and turn off MOSFET M502, the current increase in L502 isstill too slow because of its high inductance. When the voltage dropsbelow a threshold level Vi1, switches S503, S504 are switched on by theapparatus B501, B502, IC504 and IC505. Inductor L501 which has smallerinductance is connected in parallel with inductor L502. This reduces theoverall converter inductance and the current can increase rapidly asillustrated in FIG. 10D. During the time period between t32 and t33 thecurrent increases through inductor L501. This current increase raisesthe output voltage until it reaches another voltage level V12, asillustrated in FIG. 10E at time t33. Once the voltage level V12 isreached, switches S503, S504 are turned off by the apparatus B501, B502,IC504 and IC505. Current in inductor L501 is diverted through diode D504and decreases until time t34. At time t34 diode D504 is turned off andcurrent through inductor L501 diminishes to zero. During the time periodt32 to t34 the current in inductor L502 also increases towards a newvalue. If this new current value is able to sustain the output voltagefrom time t34 and after, the converter will resume normal Pulse WidthModulation with switches M501 and M502. If this new current value is notsufficient to sustain the output voltage, the output voltage will dropback to voltage level V11 and the whole process will be triggered againto boost up the output voltage. The mechanism so described provides fastcurrent increase to tackle transient load current increase in switchingpower converters.

[0072] Operation of the circuit when there is a fast transient decreasein load current is explained in terms of the waveforms illustrated inFIG. 11. During the time period t40 to t41 the converter operates withsteady load current. At time t41 the load current decreases to a lowvalue in a step as illustrated in FIG. 1C. This causes increase inoutput voltage illustrated in FIG. 11E. Even under the assumption thatfeedback circuit 504 and PWM controller 506 are fast enough to turn offMOSFET M501 and turn on MOSFET M502, the current reduction in L502 isstill slow because of its high inductance. At t42 output voltageincreases and reaches a level V13 and switches S503, S504 are triggeredto turn on through apparatus B501, B503 IC503 and IC505. Inductor L501,which has a much smaller inductance, is connected in parallel withinductor L502. This reduces the overall converter inductance and thecurrent can change rapidly as illustrated in FIG. 11D. During the timeperiod t42 to t43 the current increases in the negative sense throughinductor L501. This current reduces output voltage until it reachesanother voltage level V14 as illustrated in FIG. 11E at time t43. Oncevoltage level V14 is reached switches S503, S504 are turned off by theapparatus B501, B503, IC503 and IC505. The current in inductor L501 isdiverted through diode D503 and decreases until time t44. At time t44diode D503 is turned off and the current through inductor L501 decreasesto zero. In the time period t42 to t44 the current in inductor L502 alsodecreases towards a new value. If this new current value is able tosettle the output voltage from time t34 and after, the converter willresume normal Pulse Width Modulation with switches M501 and M502. Ifthis new current value is not low enough to settle the output voltage,the output voltage will increase again to voltage level V13 and thewhole process will be triggered again to step down the output voltage.The mechanism so described provides fast current decrease to tackletransient load current decrease in switching power converters.

[0073] In order to provide fastest transient response of the converter,a logic circuit comprising IC501, IC502, IC503, IC504, IC505 and IC506,ensure MOSFET M501 turns on under any condition when auxiliary switchesS503, S504 is triggered by B502 to turn on, or to ensure MOSFET M502turns on under any condition when auxiliary switch S503 is triggered byB503 to turn on. If switches S503 and S504 are not triggered by B503 andB502, MOSFETs M501 and MOSFET M502 will be driven by the signal from thePWM controller 506.

[0074] The present invention has been described with reference to a buckconverter topology. It would be obvious, however, to one of skill in theart to apply the invention to other converter topologies including, butnot limited to, a boost converter, a flyback converter, a forwardconverter, a push-pull converter, a resonant converter, a full bridgeconverter, a Cuk converter, a Sepic converter, a half bridge converterand other converter topologies, without departing from the spirit of theinvention. A number of embodiments that have particular utility for fasttransient applications in switching power converters have beendescribed. However, for those skilled in the art, many more embodimentscan be envisioned based on the stepping inductance principle presented,and the embodiments described herein are just a few of the embodimentsthat may be generated by those skilled in the art using the inventiondescribed herein. Having described in detail different embodiments ofthe present invention, it is to be understood that the present inventioncould be carried out with different elements and steps. The embodimentsare presented only by way of example and are not meant to limit thescope of the present invention which is defined by the following claims.

What is claimed is:
 1. A power converter comprising: an input forreceiving input power; an output for providing regulated output power;one or more switching devices coupled to said input, wherein said one ormore switching devices produce a voltage pulse train with variable pulsewidth for regulation of said output, and wherein said voltage pulsetrain also has a high voltage level and a low voltage level; atransformer comprising a plurality of windings, such that a firstwinding is coupled between said one or more switching devices and saidoutput for conduction of current in steady state, a second winding iscoupled to said input in case of a transient increase in output power,and a third winding is coupled to a low impedance in case of a transientdecrease in output power; and a control circuit that is operable tosense voltage at the output and is also operable to couple said windingsof said transformer in cases of a transient change in output power. 2.The power converter of claim 1, further comprising one or more seriescomponents in said second and third transformer winding to facilitate achange in current during a transient change.
 3. The power converter ofclaim 2, further comprising switches in series with said second andthird transformer windings for coupling to input and low impedanceelements.
 4. The power converter of claim 1, 2 or 3 further comprisingcontrol circuits that operate said one or more switching devices coupledto said input to produce a high voltage level in case of a transientincrease in output power and a low voltage level in case of a transientdecrease in output power.
 5. A power converter comprising: an input forreceiving input power; an output for providing regulated output power;One or more switching devices coupled to said input producing a voltagepulse train with variable pulse width for regulation of output, suchvoltage pulse train also having a high voltage level and a low voltagelevel; an inductor coupled between said one or more switching devicesand said output for conduction of current in steady state; a transformerhaving a plurality of windings such that a first winding is coupledbetween said switching devices and output, a second winding is coupledto said input in case of a transient increase in output power, and athird winding is coupled to a low impedance in case of a transientdecrease in output power; and a control circuit that is operable tosense voltage at the output and is also operable to couple said windingsof said transformer in cases of transient change in output power.
 6. Thepower converter of claim 5 further comprising one or more seriescomponents in said second and third transformer winding to facilitate achange in current during a transient change.
 7. The power converter ofclaim 6 further comprising switches in series with said second and thirdtransformer windings for coupling to input and low impedance elements.8. The power converter of claim 5, 6 or 7 further comprising additionalcontrol circuits that operate said one or more switching devices coupledto said input to produce a high voltage level in case of a transientincrease in output power and a low voltage level in case of a transientdecrease in output power.
 9. A power converter comprising: an input forreceiving input power; an output for providing regulated output power;an isolated DC to AC converter producing a voltage pulse train withvariable pulse width for regulation of output and such voltage pulsetrain also having a high voltage level and a low voltage level; atransformer having a plurality of windings such that a first winding iscoupled between said DC to AC converter output for conduction of currentin steady state, a second winding coupled to a low impedance element incase of a transient increase in output power, and a third windingcoupled to a low impedance element in case of a transient decrease inoutput power; and a control circuit that is operable to sense voltage atthe output and is also operable to couple said windings of saidtransformer in cases of transient change in output power.
 10. The powerconverter of claim 9 further comprising one or more series components insaid second and third winding to facilitate a change in current during atransient change.
 11. The power converter of claim 9 further comprisingone or more switches in series with said second and third winding forcoupling to said low impedance element.
 12. The power converter of claim11 further comprising additional control circuits such that in the caseof a transient increase in output power said series switches willsimultaneously turn on when the DC-AC converter produces a high voltagepulse, whereas in the case of a transient decrease in output power saidseries switches will simultaneously turn on when the DC-AC converterproduces a low voltage pulse
 13. A power converter comprising: an inputfor receiving input power; an output for providing regulated outputpower; one or more switching devices coupled to said input producing avoltage pulse train with variable pulse width for regulation of outputand such voltage pulse train also having a high voltage level and a lowvoltage level; a first inductor coupled between said switching devicesand said output for conduction of current in steady state; a secondinductor having a series switch coupled in parallel with said firstinductor wherein such second inductor has an inductance much smallerthan that of the first inductor; a control circuit that is operable tosense voltage at the output and is also operable to operate saidbi-directional switch in cases of transient change in output power;protective circuits that capture voltage overshoots produced by saidsecond inductor during transient changes.
 14. The power converter ofclaim 13 further comprising additional control circuits that operatepower converter switching devices coupled to input to produce a highvoltage level in case of a transient increase in output power and a lowvoltage level in case of a transient decrease in output power.